1. Field of the Invention
The present invention relates to the field of data storage systems, particularly to electronic controllers for controlling the operation of rotating rewritable (read/write) data storage devices.
2. Background Art
Computer systems employ data storage devices, for example, disk drives, to store various data for use by the computer system. A typical data storage device includes storage media, in which the actual data is stored, a read head, and a mechanism, such as a motor, for imparting relative motion between the storage media and the read head. The relative motion allows access to various portions of the storage media, and, in the case of certain types of media, such as magnetic media, allows for the production of signals representative of the data stored in the storage media.
In general, disk memories are characterized by the use of one or more magnetic media disks mounted on a spindle assembly and rotated at a high rate of speed. Each disk typically is round and has two surfaces of magnetic media. In a typical rotating medium as a storage system, data is stored on magnetic or magneto-optical disks in a series of concentric "tracks," with each track being an addressable area of the memory array. A read/write head is provided for each surface of each disk in the disk storage system. These tracks are accessed by a read/write head that detects variations in the magnetic orientation of the disk surface. The read/write head moves back and forth radially on the disk under control of a head-positioning servo mechanism so that it can be selectively positioned over a selected one of the tracks. Once in position over a track, the servo mechanism causes the head to trace a path that follows the center line of the selected track.
A data storage device may have several modes of operation. For example, a device may have a read mode during which data from the media is recognized and gathered, a write mode during which the stored data is written onto the media, a signal gap mode between the read mode and the write mode during which no reading or writing activity occurs, and a servo mode during which positional information is derived from the storage media for use to maintain the correct relationship between the read head and the storage media. Disk drives are used to store data by recording it on rotating media.
Data is typically stored in concentric rings known as tracks. To ensure that data may be accurately retrieved from a disk drive, the data must be stored in known locations. A number of methods have been used to provide for the correct positioning of data on a disk and for correct positioning of the read/write head relative to the data. One such method is used in an "embedded servo" disk drive. An embedded servo disk drive has positioning (servo) information recorded on the disk along with the data. The positioning information is used to control a servomechanism that then continually adjusts the position of the read/write head so that data will be read from and/or written to the disk at the proper location. This positioning information is typically located in servo gaps that occur periodically along a track. The angular distance between adjacent servo gaps is referred to as the servo interval. While reading and writing of data may occur over the servo interval, the reading and writing of user data is temporarily suspended at the beginning of a servo gap and resumed at the end of the servo gap.
In conventional systems, the position information is usually described by an ID (identification) field, followed by a data field. However, for drives with a magnetoresistive (MR) type of head, the rotary actuator used to position the MR head is known to introduce a skew angle between the MR read element and the inductive write element relative to the data tracks. A method is known to compensate the skew angle by using two ID fields, one offset with the conventional ID field. However, the disadvantage of this method is that the redundant ID field causes a commensurate loss of storage capacity (typically about 9%).
Instead of using an ID field for position information, a large servo wedge, which contains multiple bits of head and wedge information in addition to other servo fields required for correct recovery, may be used for position detection. This headerless format, which does not have ID field in front of data field, avoids the skew angle problem with an MR head. However, this approach often requires major changes to the mode in existing subsystems servo rework.
Yet another approach uses binary counters. By counting both index and sector pulses, physical wedge positions can be determined. However, this technique is usually prone to errors because the timing cannot be resolved accurately enough and hence is not a reliable method.
A current servo field usually has a single bit associated with each servo wedge to perform indexing. Among all the wedges, a bit of 1 can, for example, be assigned to a particular wedge and bits of 0 can be assigned to all other wedges. Thus, when a 1 bit is detected twice, it is then known that one complete revolution of the medium has occurred. However, this technique uniquely identifies only the single wedge having the 1 bit. The technique fails to provide unique identification about all the wedges having 0 bits.
Furthermore, more information could be added to the servo wedge. This information could be the entire header ID. This creates a problem because each bit added to the servo wedge occupies space resulting from the fact that the servo wedge is always recorded at a lower frequency than the data. This results in lost capacity saving just to implement a headerless scheme. It is desirable to save as much space in the servo wedge as possible.
Thus, a technique is needed to provide servo information and to uniquely identify data fields while maximizing storage capacity and reliability and avoiding complicated redesign of the servo system.